I want to make sure that capacity for a particular EC2 instance type in a particular region and Availability Zone is available whenever I need it. How do I purchase a Reserved Instance (RI) that does that?

In the purchasing menu, select the Only show offerings that reserve capacity box in the upper right.

Choose the desired instance characteristics from the drop-down menus.

Choose Search.

A column for Availability Zone should appear in the search results. Reserved Instances that you purchase with a specific Availability Zone will reserve that capacity for your use in that Availability Zone.